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Homework Help: Air Pressure Question

  1. Jan 19, 2006 #1
    Suppose there exists a chamber of practical size (perhaps a prism with a square-base of 10 inch size, and 25 inch height).

    Now suppose I place an open jar into the chamber.

    If (and I say 'if' because i don't know how one would do this) a machine sucked all the air from the chamber, would there exist a 'vacuum' inside the chamber? If so, would placing a lid on the jar cause the lid to 'stick' to the jar's top because of difference in air pressure?

    Please explain these concepts to me. Also, If my previous statements are true, what are the consequences (if any) of opening up this chamber after the procedure?
  2. jcsd
  3. Jan 20, 2006 #2


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    A vacuum is simply a reduced amount of gas/air molecules, and a perfect vacuum would be the complete absenced of gas molecules.

    Pressure of a gas is simply due to the kinetic energy of the gas and the transfer of momentum from the gas to a surface. As the temperature decreases (i.e. gas cools) or the density is decreased, the pressure is reduced.

    If one has a sealed container surrounded by air/atmosphere, and one withdraws the air in the container, then the pressure outside is greater than the pressure inside. The lid sealing the container will have a greater pressure on the outside surface than on the inside, and therefore, it will experience a force pushing against the lid to keep the container sealed.

    If one could exert sufficient force to open the lid, then one would hear the gas entering the container until the pressure inside and out equalized.

    See - http://hyperphysics.phy-astr.gsu.edu/hbase/press.html#pre


  4. Jan 20, 2006 #3
    If the jar is inside of the vaccuum chamber, there will be no differential pressure between the two. The lid would not be held on.

    If you put the lid on the jar, then opened the vaccuum chamber to atmosphere, the lid would definately stick on there. edit: (because the jar is sealed and is at the vaccuum pressure)
  5. Jan 20, 2006 #4
    What machine could I use to pump air out of a chamber?
  6. Jan 20, 2006 #5
    A vaccuum pump.

    or a venturi set up. Theres a few
  7. Jan 21, 2006 #6


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    As Homer mentioned, a vacuum pump, but it's a bit more complicated than that, because as the is drawing down, it becomes increasingly difficult to 'pump' out the air. One could start with a diaphragm or piston, the displacement of which increases the volume which reduces the molecular density.

    At some point, a molecular diffusion pump is necessary to remove the gas molecules.

    For an extremely high vacuum, i.e. essentially complete absence of gas molecules, a getter is used in which the oxygen atoms react with the metal in the getter to form oxide. Such a getter might be Mg or Ti or Zr. This approach also presupposes that the atmosphere is mostly oxygen, which can be achieve by pumping pure oxygen to displace air, which is of course ~78% N2, ~21% O2 and less than 1% trace gases.
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